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Forcing As a Computational Process

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FORCING AS A COMPUTATIONAL PROCESS JOEL DAVID HAMKINS, RUSSELL MILLER, AND KAMERYN J. WILLIAMS Abstract. We investigate how set-theoretic forcing can be seen as a compu- tational process on the models of set theory. Given an oracle for information about a model of set theory hM; 2M i, we explain senses in which one may compute M-generic filters G ⊆ P 2 M and the corresponding forcing exten- sions M[G]. Specifically, from the atomic diagram one may compute G, from the ∆0-diagram one may compute M[G] and its ∆0-diagram, and from the ele- mentary diagram one may compute the elementary diagram of M[G]. We also examine the information necessary to make the process functorial, and con- clude that in the general case, no such computational process will be functorial. For any such process, it will always be possible to have different isomorphic presentations of a model of set theory M that lead to different non-isomorphic forcing extensions M[G]. Indeed, there is no Borel function providing generic filters that is functorial in this sense. 1. Introduction The method of forcing, introduced by Paul Cohen to show the consistency of the failure of the continuum hypothesis, has become ubiquitous within set theory.1 In this paper we analyze this method from the perspective of computable structure theory. To what extent is forcing an effective process? Main Question. Given an oracle for a countable model of set theory M, to what extent can we compute its various forcing extensions M[G]? We answer this, considering multiple levels of information which might be given by the oracle. Main Theorem 1 (Forcing as a computational process). (1) For each countable model M of set theory and for any forcing notion P 2 M there is an M-generic filter G ⊆ P computable from the atomic diagram of M. 2 (2) Given the ∆0-diagram of M and a forcing notion P 2 M, we can uniformly compute such a G, the atomic diagram of M[G], and moreover the ∆0- diagram of M[G]. 2010 Mathematics Subject Classification. 03C57, 03E40. Key words and phrases. Forcing, computable structure theory. We thank the anonymous referee for their helpful comments. The second author was supported by NSF grant # DMS-1362206, Simons Foundation grant # 581896, and several PSC-CUNY research awards. Commentary can be made about this article on the first author's blog at http://jdh.hamkins.org/forcing-as-a-computational-process. 1We will give an overview of the construction during Section 4. For full details we refer the reader to [Sho71] or [Kun80]. See also [Cho09] for a conceptual overview. 2 ∆0 in the sense of the L´evyhierarchy, as described below. 1 2 JOEL DAVID HAMKINS, RUSSELL MILLER, AND KAMERYN J. WILLIAMS (3) From P and the elementary diagram of M, we can uniformly compute the elementary diagram of M[G], and this holds level-by-level for the Σn- diagrams. These statements are proved in Section 4 as Theorems 8, 10, and 11 below. In Section 5 we will extend these results to look at the generic multiverse of a countable model of set theory, i.e. those models obtained from the original model by taking both forcing extensions and grounds, where taking grounds is the process inverse to building forcing extensions. In Section 6 we also consider versions of these results for class forcing instead of set forcing. We remark here that in this context the ∆0-diagram of M refers to the set of formulae true in M that are ∆0 in the L´evyhierarchy. In this hierarchy, the standard one used within set theory, ∆0-formulae are those whose only quantifiers are bounded by sets, that is, of the form 9x 2 y or 8x 2 y. This differs from ∆0 in the arithmetical hierarchy, whose formulae's quantifiers are only those bounded by the order relation on !, that is of the form 9x < y or 8x < y. In Section 2 we will show that very little can be computed from the atomic diagram for a model of set theory. From the diagram we are not even able to compute relations as simple as x ⊆ y. We view this as evidence that for the computable structure theory of set theory the L´evy∆0-diagram is an appropriate choice of the basic information to be used. The usual signature of set theory|just equality and the membership relation|is too spartan to say anything of use. In Section 3 we introduce an expansion of the signature for set theory which captures the strength of the L´evy ∆0-diagram. We show that the Σn-formulae in the L´evyhierarchy are precisely those that are Σn in the arithmetical hierarchy with the expanded signature. We end the paper, in Sections 7 and 8, by investigating how much information is required to make the process of computing M[G] from M functorial. Without significant detail about the dense subsets of P, it will not be so. Recall that a pre- sentation of a countable structure M is simply a structure isomorphic to M whose domain is !. The following theorem emphasizes the importance of not conflating the isomorphism type of M with a specific presentation of M. Main Theorem 2 (Nonfunctoriality of forcing). There is no computable procedure and indeed no Borel procedure which performs the tasks of Main Theorem 1 in a uniform way so that distinct presentations of the model M will result in isomorphic presentations of the extension M[G]. We also show that forcing can be made a functorial process. One way to do this is to add extra information to the signature of the model. Another way is to restrict to a special class of models, namely the pointwise definable models. 2. The atomic diagram of a model of set theory knows very little In this section, we show that very little about a model of set theory can be computed from its atomic diagram. In particular, many basic set-theoretic relations are not decidable from the atomic diagram. As a warmup, let us first see that the atomic diagram does not suffice to identify even a single fixed element. Proposition 3. For any countable model of set theory hM; 2M i and any element b 2 M, no algorithm will pick out the number representing b uniformly given an oracle for the atomic diagram of a copy of M. FORCING AS A COMPUTATIONAL PROCESS 3 For example, given the atomic diagram of a copy of M, one cannot reliably find the empty set, nor the ordinal !, nor the set R of reals. Proof. Fix b 2 M and fix an oracle for the atomic diagram of a copy of M. Suppose we are faced with an algorithm purported to identify b. Run the algorithm until it has produced a number that it claims is representing b. This algorithm inspected only finitely much of the atomic diagram of M. The number b it produced has at most finitely many elements in that portion of the atomic diagram. But M has many sets that extend that pattern of membership, and so we may find an alternative copy M 0 of M whose atomic diagram agrees with the original one on the part that was used by the computation, but disagrees afterwards in such a way that the number for b now represents a different set. So the algorithm will get the 0 wrong answer on M . This idea can be extended to characterize which relations on M are computably from the atomic diagram. In particular, any such relation must contain both finite and infinite sets. Theorem 4. Let hM; 2M i j= ZF be a countable model of set theory and let X ⊆ M n for some 1 ≤ n < !. The following are equivalent. (In the following, Σ1 in all cases refers to the arithmetical hierarchy, not the L´evyhierarchy.3) (1) X is uniformly relatively intrinsically computably enumerable in the atomic diagram of M. That is, there is a single computably enumerable operator which given the atomic diagram of a presentation of M will output the copy of X for that presentation. (2) Membership of each single ~a in X is witnessed by a finite pattern of 2 in the transitive closures of fa0g;:::; fan−1g, and the list of finite patterns witnessing membership is enumeration-reducible to the Σ1-diagram of M. (3) There is a L!1;!-formula '(x) so that '(x) is Σ1 and defines X, and the set of (finite) disjuncts in this formula is enumeration-reducible to the Σ1- diagram of M. Proof. (1 , 3) is a standard fact in computable structure theory, first established in [AKMS89], relativized here since M does not have a computable presentation. (2 ) 1) For notational simplicity we will present the argument for the n = 1 case. Suppose membership of a in X ⊆ M is witnessed by a finite pattern of 2 in the transitive closure of fag. That is, a is in X if and only if one of a certain list of finite graphs can be found in the pointed graph (TC(fag); a; 2M ). By hypothesis this list is e-reducible to the Σ1-diagram of M, which we can enumerate, since we have the atomic diagram of M as an oracle. Therefore, we can list out these finite graphs, one by one. Meanwhile, we enumerate 2M and M, and continually check whether the most recent pair in 2M has completed a copy of a graph from this list.
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